Alastair Duncan walked into the FBI’s Queens office yesterday, thinking he was going to render an expert opinion on Tiffany glass for the agency – as he had done many times before.

Instead, he was arrested for conspiring with a grave robber to sell two large stained-glass Tiffany windows from mausoleums in New York cemeteries.

Duncan, 57, was charged in Manhattan federal court with transporting and selling stolen goods, witness tampering and hiding payments he made to the grave robber from the IRS. He was released on $100,000 bond.

According to the FBI, Duncan sold a stolen, 9-foot Tiffany window to a Japanese collector for $219,980 last November, after it had been stolen from the Salem Fields cemetery on the Brooklyn-Queens border by Anthony Casamassina.

Casamassina, who was charged in January and described as “the grave robber” in the Duncan indictment, is a former cemetery worker who lives with his mother in Middle Village, Queens.

The FBI said he kept his keys to graveyards after leaving his job and has admitted to an undercover agent that he stole stained glass, bronze urns containing human ashes and marble tables from mausoleums.

Casamassina and Duncan – introduced by a “middleman” – allegedly met Jan. 19 to discuss the sale of the pilfered window.

Duncan, who is a consultant for Christie’s auction house and the author of several books on the work of Louis Comfort Tiffany, inspected the window – which depicts a stream running through a forest – last July. Despite knowing it was stolen, Duncan went ahead with the buy after consulting the “Art Loss Register” and finding no one had reported it pilfered, the indictment said.

Duncan and the middleman each allegedly paid Casamassina $30,000. Duncan paid his share in five $6,000 checks to avoid reporting the transaction to the Internal Revenue Service, the feds charge.

Last December, Duncan asked Casamassina to “seek and find” another Tiffany window, which prompted the former cemetery worker to canvas the Kensico cemetery in Valhalla, the indictment charged.

Casamassina located a stained glass window in a Kensico mausoleum, then showed Duncan a photo of it. But, Duncan said, “his guy needed a window that was larger and had more trees,” the indictment said.